Up till now, preforms of this kind have been manufactured by pelletizing under high pressure.
When pelletizing under high pressure, the mineral insulating substance is dampened before being placed in a die which is then subjected to high pressure. The preforms obtained have ducts through which well centered conductors pass, but these conductors are in very short lengths (about one centimeter). Only about 2 to 16 m per hour can be produced by this discontinuous method. These elements are then inserted one after another in the tube which is to sheath them. This is a relatively long operation.
The other method of filling tubes consists in placing the mineral insulating substance in powder form in a vertically disposed metal tube fitted with its conductor(s) while compressing the powder. This method does not provide proper centering of the conductors nor such compactness as the previous one, especially during initial cable-drawing operations. Only relatively short lengths of cable can be produced by this method.
Preferred embodiments of the present invention overcome the disadvantages of both known methods and allow partially continuous preform manufacture with appreciably faster insertion in the tubes than with the pelletizing under high pressure method. Production may reach 250 m per hour under favourable conditions. The present invention can also provide good centering of the conductor-housing ducts.